1. Field of the Invention
The present invention relates to a diffractive optical element which can have a high diffraction efficiency in a visible wavelength region, and which is suitable for use in an optical apparatus, such as a video camera or a digital camera.
2. Description of the Related Art
As a method of reducing chromatic aberration of an optical system, a method of combining different lens materials is known. In addition, a method using a diffractive optical element in an optical system is known. Refer to SPIE Vol. 1354 International Lens Design Conference (1990), and to U.S. Pat. No. 5,044,706.
A diffractive optical element not only can correct chromatic aberration, but also can provide an aspherical effect as a result of appropriately changing a period of a periodic structure.
In an optical system including a diffractive optical element, when a light beam in a usable wavelength range is mostly diffracted light of a particular order (design order), the intensity of diffracted light of orders other than the particular order is reduced.
Diffracted light of orders other than the design order propagates in a direction differing from that of design-order light and thereby becomes flares.
Therefore, when a diffractive optical element is to be used in an optical system, the diffraction efficiency of the design-order diffracted light needs to be sufficiently high over the entire usable wavelength range.
FIG. 15 is a sectional view of the main portion of a known diffractive optical element (single-layer DOE) comprising a substrate 302 and a diffraction grating 301 formed on the substrate 302.
FIG. 16 is a graph showing diffraction efficiency with respect to particular orders of the single-layer DOE of FIG. 15.
In FIG. 16, the horizontal axis represents the wavelength of incident light, and the vertical axis represents the diffraction efficiency. The diffraction efficiency refers to the ratio of the quantity of diffracted light for each order with respect to the quantity of transmitted light. However, reflected light at a boundary surface of the grating is not considered.
As shown in FIG. 16, the single-layer DOE shown in FIG. 15 is designed so that the diffraction efficiency of first-order diffracted light is highest. That is, the design order corresponds to the first order.
The diffraction efficiency of the first-order diffracted light is highest at a certain wavelength (corresponding to a design wavelength), and gradually decreases at other wavelengths.
The diffraction efficiency for the design order is low, thereby producing zeroth-order and second-order diffracted light. This produces flares in an optical system.
A structure that reduces such flares is known. Refer to US Nos. 2004/0263982, 2001/0015848, 2004/0104379, and 2003/0231396, and Japanese Patent Laid-Open Nos. 2006-220689, 2005-1319, 2006-220816, and 2006-235007.
FIG. 17 is a sectional view of the main portion of a diffractive optical element according to US No. 2004/0263982.
In the diffractive optical element shown in FIG. 17, grating portions having two different grating thicknesses d1 and d2 and three different types of grating materials 306 to 308 are optimally selected, and a plurality of diffraction gratings are disposed in contact with each other at an equal pitch distribution.
Accordingly, as shown in FIG. 18, a high diffraction efficiency is provided for the design order over the entire visible range.
FIG. 19 is a sectional view of the main portion of a diffractive optical element according to US No. 2001/0015848.
The diffractive optical element shown in FIG. 19 has a structure (stacked DOE 201) in which element portions 202 and 203, each including a diffraction grating, are disposed close to each other with an air layer 210 being disposed therebetween. By optimizing, for example, the thicknesses of the grating portions of the respective layers, dispersion characteristics (Abbe number νd), and refractive indices of the materials of the respective diffraction gratings, a high diffraction efficiency is provided for the design order over the entire visible range as shown in FIG. 20.
In diffractive optical elements according to US Nos. 2004/0104379 and 2003/0231396, ITO fine particles, etc., are used in materials of diffraction gratings to properly set dispersions and refractive indices of the materials. Accordingly, diffractive optical elements having little flare even in various image-forming conditions while providing a high diffraction efficiency are realized.
In general, materials using ITO fine particles easily provide a high diffraction efficiency. However, as discussed in Japanese Patent Laid-Open Nos. 2006-220689, transmittance tends to be reduced due to coloration of the ITO fine particles. In addition, it is difficult for the fine particles to have dispersibility, and light scattering tends to increase.
According to Japanese Patent Laid-Open No. 2006-220689, a diffractive optical element having a high transmittance while having a high diffraction efficiency is obtained as a result of properly setting an optical material of a diffraction grating.
When producing a diffractive optical element using a material in which transparency of ITO fine particles, etc., is low and in which light scattering tends to occur, it is possible to improve transmittance and reduce light scattering if, for example, the thickness of an optical layer comprising a base and a grating portion of a diffraction grating is reduced.
However, when the thickness of the optical layer is reduced, a wall thickness ratio of the optical layer is increased. Therefore, for example, peeling, cracking, and distortion during molding tend to occur. Further, stability with respect to environmental changes after manufacturing tends to be reduced.
Japanese Patent Laid-Open Nos. 2005-1319 and 2006-220816 each propose a molding method that can increase the stability during molding by concentrating changes in shape at a free surface that does not contact a mold.
Japanese Patent Laid-Open No. 2006-235007 proposes a molding method using an intermediate layer disposed between an optical layer and a substrate and formed of a material differing from that of the optical layer.
In general, when a diffraction grating is formed on a substrate, a grating base (formed of a material that is the same as that of a grating portion) is provided between the diffraction grating and the substrate. In addition, the thickness of the grating base is increased to reduce a wall thickness ratio of the grating portion, so that, for example, distortion during molding is prevented from occurring.
When ITO fine particles are used in the material of the diffraction grating, a high diffraction efficiency is easily obtained, and it is easier to properly set dispersion.
Since ITO fine particles have high absorption and cause high light scattering, the grating base must be made as thin as possible. However, when the grating base is made thin, the wall thickness ratio is increased. Therefore, the shape stability during and after molding is reduced.
For shape stability, an intermediate layer may be provided between the substrate and the diffraction grating. However, when a plurality of layers are formed on the substrate using two or more types of materials, differences between the refractive indices of the materials and the state of an interface greatly influence the diffraction efficiency. When the difference between the refractive indices of the materials is large, scattering at the interface becomes large. In addition, when the interface is not flat, a wave surface is distorted, thereby reducing the diffraction efficiency.